CHAPTER XXXIII. 



SILURIAN GROUPS OF USK AND MAY HILL. 



1 . Usk Valley of Elevation, consisting, like that of Woolhope, of the Ludlow, Wenloch 



and Caradoc Formations. (PL 36. figs. 23 and 24.) 



2. Prolongation of the axis of Woolhope, by May Hill to Purton and Tortworth 



in Gloucestershire. (PL 36. figs. 11, 12, 13, 14, 14 bis, 15 and 16.) 



BEFORE we trace the remarkable anticlinal line 1 , which proceeding from the south- 

 eastern end of the valley of Woolhope, passes by May Hill into districts east of the 

 Severn, let us first dispose of the only Silurian tract to the west of that river, which 

 has not been described. 



This tract comprises a group, which rising from beneath the Old Red Sandstone of 

 Monmouthshire, near the town of Usk, constitutes an irregularly shaped elliptical valley 

 of elevation, of about eight miles in length by four in width ; the major axis trending 

 from Clytha on the N.N.E., to Llangibby and Llandegfydd on the S.S.W. The group 

 is bisected by the river Usk, which after traversing the Old Red Sandstone, enters these 

 rocks by a fissure at the Chain Bridge, having the Cliff called Craig pandu on the 

 one hand, and the lower grounds of Kemeys and Trostrey on the other. After winding 

 in a denudation, the river again exposes similar rocks in the Hills of Llancayo and the 

 Castle of Usk on the left bank, and in the ridges of Llanbadock upon the right. 



Ludlow Rocks. 



The Ludlow Rocks are very largely developed upon the left bank of the river, and form the 

 hills of Usk Castle, Llancayo, Trostrey and Clytha. At Usk Castle is a clear section of the 

 Upper Ludlow Rock, showing its passage into the Old Red Sandstone ; and the lower perishable 

 strata of the same formation are well exposed on the east side of the high road leading to Aber- 

 gavenny. The flaglike beds of the Upper Ludlow have the faces of their joints occasionally dis- 

 coloured with a deep red tint, which I have never observed in any other portion of the formation, 

 the purple or black oxide being the prevailing ore of iron which stains these rocks in Shropshire. 



1 The southern portion of this anticlinal is marked in the map accompanying the memoir of Dr. Buckland 

 and Mr. Conyheare. (Geol. Trans, vol. i. pi. 38.) 



